Phenol and ketones of general formula R1COCH2R2 (I), in which R1 and R2 each independently represent an alkyl group having from 1 to 4 carbon atoms, such as methyl ethyl ketone, are important products in the chemical industry. For example, phenol is useful in the production of phenolic resins, bisphenol A, ε-caprolactam, adipic acid, alkyl phenols, and plasticizers, and methyl ethyl ketone is widely used as a lacquer, a solvent and for dewaxing of lubricating oils.
A common route for the production of methyl ethyl ketone (MEK) is by dehydrogenation of sec-butyl alcohol (SBA), with the alcohol being produced by the acid-catalyzed hydration of butenes. Commercial scale SBA manufacture by hydration of butylene with sulfuric acid has been accomplished for many years via gas absorption/liquid extraction. Improvements to this hydration process include a process configuration that utilizes a unique combination of plug flow, bubble column, and CSTR (Stirred Tank Reactor) reaction sections to achieve high conversion of butylene. Other improved processes use spargers, custom-designed for butylene/sulfuric acid absorption/extraction. Also, loop reactors may be preferred to improve mixing intensity. In sec-butyl alcohol dehydrogenation, crude sec-butyl alcohol is recovered in absorption or extraction sections using several towers, preferably, a single tower, to separate sec-butyl alcohol from sec-butyl ether.
Currently, the most common route for the production of phenol is the Hock process. This is a three-step process in which the first step involves alkylation of benzene with propylene to produce cumene, followed by oxidation of cumene to the corresponding hydroperoxide and then cleavage of the hydroperoxide to produce equimolar amounts of phenol and acetone. However, the world demand for phenol is growing more rapidly than that for acetone. In addition, the cost of propylene relative to that for butenes is likely to increase, due to a developing shortage of propylene. Thus, a process that uses butenes or higher alkenes instead of propylene as feed and co-produces MEK or higher ketones rather than acetone may be an attractive alternative route of the production to phenol.
It is known that phenol and MEK can be produced from sec-butylbenzene by the Hock process, where sec-butylbenzene (SBB) is oxidized to obtain sec-butylbenzene hydroperoxide (SBBHP) and the peroxide is decomposed to the desired phenol and methyl ethyl ketone. An overview of such a process is described in pages 113-244 and 261-263 of Process Economics Report no. 22B entitled “Phenol”, published by the Stanford Research Institute in December 1977.
Methods for making phenol and MEK or higher ketones by oxidation of alkylbenzenes have also been described in several other documents.
U.S. Pat. No. 5,298,667 and EP-A-548,986 disclose a process for producing phenol and MEK which comprises the steps of (I) oxidizing one material selected from (A) sec-butylbenzene substantially free from ethyl hydroperoxide, carboxylic acids and phenol, (B) sec-butylbenzene substantially free from styrenes, and (C) sec-butylbenzene substantially free from methylbenzyl alcohol, to obtain sec-butylbenzene hydroperoxide, with an oxygen-containing gas and in the absence of a catalyst, and (II) decomposing the sec-butylbenzene hydroperoxide to obtain phenol and MEK with an acidic catalyst.
EP-A-1,088,809 discloses a process for producing phenol, MEK and acetone by the oxidation of a mixture containing cumene and up to 25 wt % sec-butylbenzene and the subsequent Hock cleavage of the hydroperoxides, so that the ratio of the phenol:acetone:MEK in the product can be controlled via the composition of the feed mixture. The feed mixture is produced directly by the alkylation of benzene with a corresponding mixture of propene and 1-butene/2-butene in the presence of a commercial alkylation catalyst such as AlCl3, H3PO4/SiO2 or a zeolite. Oxidation takes place in the presence of air or oxygen and in the absence of a catalyst.
FR-A-2,182,802 discloses a process for producing phenol and MEK by oxidation of sec-butylbenzene, in which sec-butylbenzene is oxidized to sec-butylbenzene hydroperoxide in the presence of air and optionally in the presence of sec-butylbenzene hydroperoxide, followed by peroxide decomposition. According to this document, sec-butylbenzene must not contain more than 1 wt. % isobutylbenzene, as isobutylbenzene significantly affects the overall process efficiency and yield in phenol and methyl ethyl ketone.
U.S. 2004/0162448 and U.S. 2004/0236152 disclose processes for producing phenol and acetone and/or MEK, in which a mixture of cumene and sec-butylbenzene is oxidized to the corresponding peroxides in the presence of oxygen, followed by peroxide decomposition. The oxidation mixture may also contain cumene hydroperoxide as initiator, but does not contain any catalyst. According to these documents, the addition of a neutralizing base in the oxidation mixture improves the yield in hydroperoxide and reduces the formation of undesired side products.
U.S. Pat. No. 6,852,893 and U.S. Pat. No. 6,720,462 describe methods for producing phenol by catalytic oxidation of alkyl aromatic hydrocarbons to the corresponding hydroperoxide, and subsequent cleavage of the hydroperoxide to give phenol and a ketone. Catalytic oxidation takes place with oxygen, in the presence of a free radical initiator and a catalyst, typically an N-hydroxycarbodiimide catalyst, such as N-hydroxyphthalimide. Preferred substrates that may be oxidized by this process include cumene, cyclohexylbenzne, cyclododecylbenzene and sec-butylbenzene.
Oxydation of alkylbenzenes with N-hydroxyphthalimide is also mentioned in Y. Ishii et al., J. Org. Chem. 1995, 60, 3934-3935; EP-A1-858835; EP-A1-824962; U.S. Pat. No. 6,476,276 and JP-A-2003-034679 as well as in A. Burghardt et al., Chemia Stosowana 1979, XXIII, 4, 443-458, and in R. A. Sheldon et al., Adv. Synth. Catal. 2004, 346, 1051-1071.
In comparison to cumene, oxidation of aromatic compounds substituted by branched alkyl groups having 4 or more carbon atoms, such as sec-butylbenzene, to the corresponding hydroperoxide requires higher temperatures and is very sensitive to the presence of impurities. For example, at about 110° C. cumene easily undergoes atmospheric air oxidation, while sec-butylbenzene does not undergo any significant oxidation. Atmospheric air oxidation of sec-butylbenzene requires higher temperatures than cumene oxidation, with the inconvenience that higher temperatures lead to poor selectivity to the desired phenol and ketone products.
Without wishing to be bound by any theory, it is believed that branched alkyl substituents on the benzene ring, having 4 or more carbon atoms, can undergo carbon-carbon bond scission at the beta position from the benzene ring under oxidation conditions, thereby generating alkyl radicals that terminate radical chain propagation and preventing oxidation to take place.
Furthermore, certain by-products formed during alkylbenzene manufacture typically present in alkylbenzenes where the alkyl group has 4 carbon atoms or more also inhibit alkylbenzene oxidation.
These drawbacks have, up to now, limited the use of the Hock reaction to make phenol from alkylbenzenes in which the alkyl chain has 4 or more carbon atoms. There thus remains a need to find new alkylbenzene oxidation conditions that are much less sensitive to the presence of impurities than the existing oxidation processes, and that allow selective and efficient commercial scale production of phenol and ketones of formula (I) by the Hock process.